Rheology: the chemistry of particles

Size and Shape

Particle size and surface chemistry are critical to the rheological behaviour of a slurry. As particle
size decreases, surface effects such as dispersion and flocculation become increasingly of practical concern. Similarly with decreasing particle size, those factors which affect surface behaviour, such as surface charge and adsorbed species, become increasingly significant. Some typical particle sizes and the physical model for their behaviour are given below in Table 1.

Table 1: Particle Size and Surface Chemistry

Size(diam)

Area/Mass

Example

Behaviour

1000 um

0.1

coarse sand

Newtonian

100

1

fine sand

…

10

10

coarse clay

Stokesian

1

100

clay

…

0.1

1000

milk

Colloid

Surface effects dominate in particles of 1 micron or less, and surface forces are often significant for particles up to 20 microns in diameter.

Particle shape is also important in determining the rheology
of a suspension or slurry. In the absence of other differences, spherical particles will interact less than plate-like or needle-like particles, and produce slurries of lower viscosity.

Surface Charge

The effective surface charge of particles primarily determines their dispersion and aggregation. This effective charge is measured as the zeta potential, which is derived from the actual surface charge modified by the molecules and ions which are dragged along with a particle as it moves in a solution.

The zeta potential is a measure of the charge at the moving boundary, or “shear plane” (see diagram below) which exists in the solution close to the moving
particle. The zeta potential depends upon the concentration of ions in the solution, the pH of the solution, and the presence or otherwise of ions, especially multivalent ions.

The ionic concentration affects the zeta potential by causing more of the surface charge to be neutralised in the solution close to the surface of the particle. The effective surface charge decreases rapidly as ionic concentration increases, up to about 0.1 M depending on the ions involved.

Effective surface charge can also be modified by pH, since almost all particles contain surface species or functional groups which cause them to act as acids or bases. Therefore there is
some pH at which the overall charge of the particle, including its “passenger” ions is zero. This pH is the particles isoelectric point. Most inorganic and organic particles have isoelectric points. This includes minerals, polymers, proteins and even bacteria.

Multivalent ions preferentially adsorb at oppositely charged surfaces and greatly reduce a particles effective surface charge. Particles tend to coagulate when their effective surface charge is close to zero. This is because under these conditions the electrostatic forces of repulsion are very small.